Variable-speed belt drives are commonly used on small vehicles such as snowmobiles, scooters or small cars. Such belt drives substantially comprise a driving pulley, a trapezoidal belt and a driven pulley. The driving pulley is linked to an engine and the driven pulley is usually mechanically connected to ground traction means, such as wheels or tracks.
The main object of using a variable-speed belt drive is to automatically change the winding diameter of the driving and the driven pulleys in order to have a maximum torque at low speeds and a reasonable engine rotation speed at high speeds. The sides of the trapezoidal belt are, on each pulley, gripped between two opposite flanges wherein one is fixed and one is movable. At low speeds, the winding diameter of the driving pulley is small and the winding diameter of the driven pulley is maximum. As the rotation speed of the driving pulley increases, sliding flyweights, located between radially converging ramps facing each other, are subjected to an increasing outward force pushing the ramps away from each other. This axial thrust generated by the flyweights is counterbalanced by biasing means, such as a helicoidal spring mounted around the shaft of the driving pulley, which biases the movable flange away from the fixed flange. As the axial thrust pushes the movable flange towards the fixed flange, the spring is further compressed. The movement of the movable flange stops when the axial thrust becomes substantially equal to the opposite biasing force.
When the rotation speed of the engine decreases, the winding diameter of the driving pulley decreases since the movable flange of the driving pulley moves back away from the fixed flange because the biasing force is then more important than the axial thrust of the flyweights. The movement of the movable flange stops when the axial thrust becomes substantially equal to the opposite biasing force.
An example of a driving pulley in a variable-speed belt drive is disclosed in U.S. Pat. Nos. 3,659,470 and 5,052,981, and also in Canadian Patent No. 1,212,559.
One of the drawbacks of conventional driving pulleys is that the flyweights may have problems when following a curved contact surface of a ramp because the contact point moves as the flyweights are displaced. This affects the normal function of the driving pulley and alters the calibration thereof.